No, not the sensation you get when you have gulped your ice cream too fast. SUGAR Freeze is the a new propulsion concept developed by Boeing that aims to revolutionize air travel. Standing for Subsonic Ultra Green Aircraft Research, the NASA-commissioned project (codenamed "N+4") looks at immature technologies in the hope of kickstarting research for the future. It's reportedly 60 percent more efficient than the equivalent Boeing 737-800, thanks to a very experimental propulsion system. Cryogenically stored liquified natural gas (hence "Freeze") is burned in a pair of unducted fan engines while also powering a solid-oxide fuel cell as an aft-thruster. With LNG projected to remain abundant, more environmentally friendly and cheap well into the century, it makes an ideal substitute to current aviation fuel, which is none of those things. Currently it's far too unsafe a design to contemplate building, and there are concerns about the natural gas production process, but hopes remain that the kinks will be ironed out well before the 2045 deadline.